Group III nitride-based semiconductors are widely used for efficient blue and ultraviolet light emitting diodes, lasers, ultraviolet detectors, and field effect transistors. Due to a wide band-gap, group III nitride semiconductor materials are one of the prime choices for deep ultraviolet light emitting diodes (DUV LEDs). While significant improvements in efficiency have been made for DUV LEDs in recent years, the overall efficiencies of these devices remains low. For example, the wide band-gap of the group III nitride semiconductor materials makes it difficult to form a good ohmic contact to the semiconductor layers, which leads to resistive losses at the contact junction.
DUV LED devices frequently employ flip-chip technology to control light extraction and thermal management of the DUV LED devices. For example, FIG. 1 shows a typical design of a flip-chip LED 2 according to the prior art. In this design, most of the light generated in the active region 4 is extracted through a transparent substrate. Efficiency of the flip-chip LED 2 is heavily dependent on the transparent properties of the semiconductor layers. However, for the flip-chip LED 2 to have a high efficiency, it also is desirable for each of the p-type contact 6 and the n-type contact 8 to be both ohmic and reflective, thereby allowing each contact 6, 8 to serve as an electrode as well as a mirror for reflecting light emitted by the active region 4. Additionally, the contacts 6, 8 should have stability during thermal cycling while packaging, as well as during operation of the device. Aluminum is a good reflecting metal, however, it does not produce an ohmic contact and is unstable during packaging.
Several types of metallic contacts have been proposed to improve ohmic contact to semiconductor layers. These contacts are formed of, for example: nickel/gold (Ni/Au), cobalt/gold (Co/Au), palladium/gold (Pd/Au), rhodium (Rh)-based, palladium/platinum/gold (Pd/Pt/Au), Pt/Ni/Au, Ni/Pt/Au, Ni/Pd/Au, and titanium/platinum/gold (Ti/Pt/Au) metallic layers. The thermal stability of Pd/Ni contacts is enhanced due to the formation of Pd gallides. Additionally, Pd/Ni contacts can lead to a reduction of contact resistivity. For Ni-based contacts, the Ni is easily oxidized above 400 degrees Celsius and the ohmic contact becomes worse at temperatures above 500 degrees Celsius.
One approach proposed a Pd/Ni/Al/Ti/Au metallization scheme for a contact, with the layers having corresponding thicknesses of 3 nanometers (nm)/2 nm/150 nm/20 nm/30 nm. The contact exhibited good thermal stability, reflectivity of sixty-two percent for radiation having a wavelength of 370 nm at normal incidence, and good ohmic characteristics after annealing at three hundred degrees Celsius in nitrogen gas (N2). It is further noted that the combination of Pd and Ni results in a good ohmic contact, while a contact without Ni results in a larger resistance and non-linear behavior. Another approach removes residual oxide from a gallium nitride (GaN) surface using Ni deposition to achieve better ohmic contact properties.
To date, most contact engineering has been for visible LEDs or near ultraviolet (UV) LEDs. For example, one approach found good ohmic properties for an iridium/silver (Ir/Ag) p-type contact with a seventy-five percent reflectivity for radiation having a wavelength of 405 nm. For the same radiation wavelength, an indium-doped zinc oxide/silver (ZnO/Ag) contact had a reflectivity of 82.3%. These are only illustrative of many contact schemes proposed for radiation in the near UV wavelength. Indium tin oxide (ITO) and ZnO contact also have been proposed for LEDs operated at near UV or UVA wavelengths. However, for UV LEDs different contacts are required in order to provide highly reflective UV mirrors.
To lower resistance of a Schottky barrier, a difference between the work function of the metal and the semiconductor can be reduced. Unfortunately, for group III nitride semiconductors, the band gap is large and the resulting work function for the p-type semiconductors is large as well. It is understood that not only the work function of an aluminum gallium nitride (AlGaN) and metallic contact determines the behavior of the Schottky junction for a p-type semiconductor. The presence of high density surface states for covalent semiconductors pins the Fermi level at the interface. Regardless, it has been observed that the ohmic contact is sensitive to the metal work function. For example, aluminum, with a low work function of approximately four electron volts (eV) does not result in an ohmic contact. However, Pd and Ni, each with a work function above five eV form better ohmic contacts to semiconductor materials.
An approach describes a reflective electrode for a semiconductor light emitting device as including an ohmic contact layer formed of Ag or an Ag-alloy, which forms an ohmic contact with a p-type compound semiconductor layer. The Ag-alloy can be an alloy of Ag and a group of materials, such as magnesium (Mg), Zn, scandium (Sc), hafnium (Hf), zirconium (Zr), tellurium (Te), selenium (Se), tantalum (Ta), tungsten (W), niobium (Nb), copper (Cu), silicon (Si), Ni, Co, molybdenum (Mo), chromium (Cr), manganese (Mn), mercury (Hg), and praseodymium (Pr). The contact can include a layer composed of Ni or an Ni-alloy, which can have a thickness in the range between 0.1 and 500 nm. The contact also includes a layer located on the ohmic contact layer or the Ni layer, which is formed of a material selected from: Ni, Ni-alloy, Zn, Zn-alloy, Cu, Cu-alloy, ruthenium (Ru), Ir, and Rh, and a subsequent layer formed of a light reflective material. The reflective material can be Ag, Ag-alloy, Al, Al-alloy, or Rh, and have a thickness between 10 to 5000 nm. The electrode can further include another layer on the light reflective material to prevent an agglomeration phenomenon during the annealing process, which can occur on the surface of the reflective material without the additional layer being present. The layer can be formed of a material selected from: Cu, Cu/Ru, Cu/Ir, Cu-alloy, Cu-alloy/Ru and Cu-alloy/Ir.